The problems associated with choosing communication paths through a network that includes moving nodes and delivering communications through such paths are severely felt in networks that include satellite switching nodes. Satellite communication networks benefit from placing satellites in low earth orbits. In low earth orbits, the area of radio coverage for any single satellite is smaller than with higher orbits. The low earth orbits limit interference between communications taking place in various areas of coverage and allow a spectrally efficient cellular communication system that uses smaller cells.
However, satellites in low earth orbits move relative to the surface of the earth. Typically, communications take place between two or more points near the surface of the earth. A communication path that includes particular satellite nodes may be advantageous one moment and disadvantageous the next due to satellite movement. Accordingly, as one satellite disappears from the radio view of a point near the surface of the earth, a handoff operation should switch a communication link between the point and the satellite to some other node of the network so that communications may continue.
In a satellite communication network, this other node may be another satellite. In fact, the network may arrange an entire constellation of satellites in low earth orbits so that as one satellite departs the radio view of a point near the surface of the earth, another satellite arrives. Desirably, some overlap occurs so that communications may continue with the departing satellite until a communication link is established with the arriving satellite. On the other hand, this overlap is no larger than necessary to perform a handoff because the tremendous expense associated with placing and maintaining satellites in orbit demands a constellation of no more satellites than is minimally required.
Orbiting satellites move in predictable orbits. Positions of various satellites relative to a point near the surface of the earth may be calculated well in advance of the time when the satellites will occupy those positions. On the other hand, precisely predicting the duration required for a land station to acquire an arriving satellite's signal is a difficult task. Environmental factors, such as the potential presence of rain in the vicinity, can affect acquisition time, along with random occurrences related to scanning for and synchronizing with the arriving satellite's signal.
A satellite communication network could simply budget for failed handoffs when significant rain is in the vicinity. However, the tremendous expense associated with a satellite communication network makes such rain-based unreliability intolerable. Alternatively, a satellite communication network could design the satellite constellation so that a failure to acquire a satellite in a worst case rain situation is extremely unlikely. However, this requires greater overlap time between satellites, wastes scarce resources for the vast majority of situations, and increases costs significantly.